The invention relates to a process for the preparation of enantiomers of optically active dicarboxylic acid monoesters by intramolecular rearrangement.
Enantiomers of optically active dicarboxylic acid monoesters are valuable intermediate products in the preparation of optically active compounds, such as, for example (-)-pantolactone (Seebach, Wasmuth; Helv. Chim. Acta 63 (1980), 197) or the pheromone anthopleurin (Musich, Rapoport; J. Amer. Chem. Soc. 100 (1978), 4865).
A number of processes for the preparation of enantiomeric dicarboxylic acid monoesters are known. Thus, dicarboxylic acid diesters can be partially hydrolyzed by chemical means (for example Durham et al. in Org. Synth. Coll. Vol. IV, Wiley, New York, 1963, p. 635). Enzymatic processes of hydrolysis, as disclosed, for example, in EP-A 0,084,892 and JP-A 57,198,098, afford an enantiomer directly. Monoesters are also obtained from dicarboxylic anhydrides by alcoholysis (Houben-Weyl, Methoden der Organischen Chemie ("Methods of Organic Chemistry"), Georg-Thieme-Verlag, Stuttgart, Volumes VIII+E5). The dicarboxylic acid monoesters obtained by the above processes can be resolved into the optical antipodes by known methods for resolving racemates (for example in P. Newman: Optical Resolution Procedures for Chemical Compounds, Vol. 2, Optical Resolution Information Center, Manhattan College, Riverdale, N.Y., 1981), for example via the formation of diastereomeric salts, as described in EP-A 0,092,194 or DE-A 3,431,294.
These procedures known up to the present time exhibit a number of disadvantages. Thus, at the best only half of the total amount of desired enantiomer employed in the resolution process can be obtained from the racemic mixture of the enantiomeric dicarboxylic acid monoesters. The undesired enantiomeric which remains must be reconverted into a racemic form accessible to optical resolution. This can be effected, for example, by esterification to give the corresponding dicarboxylic acid diester, which can be partially hydrolyzed again. Furthermore, the undesired, optically active half-ester can be completely saponified to give the dicarboxylic acid, and the latter, after the conversion into the anhydride, can be converted again into a mixture of optically resolvable half-esters by means of an alcohol. The complete conversion of the undesired enantiomer into the desired enantiomer requires the processes of optical resolution and reconversion into a racemate form to be carried out, in some cases, several times.
There is therefore a need for a process which effects the conversion of optically active half-esters of dicarboxylic acid into one another without additional process stages and thus permits enantiomers of these half-esters to be prepared in a simple manner.